1. Field of the Invention
The present invention is generally related to amorphous propylene-ethylene copolymers and processes for producing such copolymers. More particularly, the present invention is generally related to amorphous propylene-ethylene copolymers having desirable needle penetrations, softening points, viscosities, and viscoelastic characteristics.
2. Description of the Related Art
Amorphous polyolefins are commonly used in industry to produce a wide array of products including, for example, adhesives. Common polyolefins utilized in adhesives generally include copolymers produced from propylene, ethylene, and various C4-C10 alpha-olefin monomers, such as, for example, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, and 1-decene. In particular, propylene-butene copolymers are commonly used to produce hot melt adhesives due to the higher adhesive bond strengths derived from these copolymers. Much of the adhesive bond strength derived from these copolymers can be attributed to the C4-C10 alpha-olefins contained therein, which can greatly increase the subsequent bonding properties of the copolymer. Unfortunately, C4-C10 alpha-olefins can be quite expensive due to market availability and can also exhibit limited reactivity during the polymerization processes.
Due to the above deficiencies of the C4-C10 alpha-olefins, some manufacturers have attempted to replace C4-C10 alpha-olefins with ethylene. Unlike many of the C4-C10 alpha-olefins, ethylene can be more readily available and more reactive than many of the commonly used C4-C10 alpha-olefins, such as 1-butene. Unfortunately, propylene-ethylene copolymers can exhibit deficiencies in hardness, thereby resulting in adhesives that lack ideal bond strength over time. Some manufacturers have attempted to increase the hardness of these copolymers by incorporating crystalline polypropylene therein. However, by adding crystalline polypropylene to these copolymers, the softening points of the copolymers are also increased. This can limit the application of these copolymers to certain types of adhesives due to the higher softening points.
Thus, there is a need for amorphous copolymers that exhibit an ideal balance between hardness and softening point and that can also be used to produce adhesives with improved adhesive characteristics.